1. Field of the Invention
The present invention relates to an LED indicator lamp, particularly to an LED indicator lamp used in traffic signals.
2. Description of the Related Art
As light emitting diodes capable of emitting light of R, G and B primary colors, and light emitting diodes capable of emitting white light with high luminance, have been developed, LED indicator lamps constituted from a plurality of light emitting diodes arranged in an array have been put in use for various applications. An LED indicator lamp has a far higher service life than that of an incandescent lamp, and also shows a high efficiency and a high resistance against vibration. For these advantages, the LED indicator lamp has been used in advertising sign boards, traffic sign boards displaying route guide or traffic information, light source for traffic signals and large screens.
With regard to an application for traffic signals, in particular, while the incandescent lamp used as a light source of a conventional traffic signal requires large reflector mirrors and color filters, the LED indicator lamp has such advantages as a capability to emit light of a single color that eliminates a need for a color filter, and a capability to emit light with some degree of directivity that eliminates a need to install a large reflector mirror.
Moreover, a traffic signal constituted from LEDs that does not need reflector mirrors and color filters also has an advantage of being free from spurious lighting, that is otherwise caused by extraneous light that has entered a traffic light, is reflected from a reflector mirror placed behind an incandescent lamp and comes out of the traffic signal through a color filter.
A constitution of a traffic signal using light emitting diodes is disclosed in U.S. Pat. No. 6,019,493, wherein a high efficiency light emitting element capable of uniform light emission is constituted by providing a lens made by integrally forming a central convex lens and a plurality of annular convex lenses located around the central convex lens.
International Patent Application PCT/IB97/01974 (International Publication No. WO98/16777) discloses an LED indicator lamp that has a convex lens (Fresnel lens) placed in front thereof, and a plurality of light emitting diodes distributed densely around the optical axis of the lens so that failure of one of the light emitting diodes does not cause significant change in a light intensity distribution.
An LED indicator lamp used in traffic signals and sign boards is usually installed at overhead height so as to be recognized by many people from a distance. As such, the LED indicator lamp is required to emit light with horizontally symmetrical intensity distribution but asymmetrical intensity distribution in a vertical direction so that light intensity is higher in a front field and lower field.
As it has been made possible to increase luminous intensity of light emitting diodes recently, it is enabled to decrease a number of light emitting diodes required in an LED indicator lamp.
However, a new problem has arisen in that it is difficult to achieve planar light emission of uniform intensity with an LED indicator lamp consisting of a small number of light emitting diodes that have high luminous intensity.